FE022: Handle screen crashes gracefully

[bhanuja4776]

Summary

• add an app-wide React error boundary around routed screen content
• show a token-based fallback with a Try again action that returns to Scan
• log the original error and React component stack for diagnosis
• preserve Expo's development red error overlay by rethrowing in development

Validation

px eslint components/errorHandling/AppErrorBoundary.tsx app/_layout.tsx

The repository-wide lint and type checks still report pre-existing errors outside this ticket.